1. Field of the Invention
The present invention relates to a double redundancy electro hydrostatic actuator system with a dual tandem hydraulic cylinder driven by using two systems of hydraulic circuits.
2. Description of Related Art
A double redundancy electro hydrostatic actuator system for controlling a dual tandem hydraulic cylinder by two systems of hydraulic circuits is known. Such a double redundancy hydrostatic actuator system is adopted in a wing of an airplane. That is, two systems of hydraulic circuits are provided to allow a dual tandem hydraulic cylinder to be operated, even when either of the two systems of hydraulic circuits does not operate.
FIG. 1 shows a conventional hydraulic actuator system. Two systems (A system and B system) of hydraulic pressure circuits are connected to the hydraulic cylinder 18. Each system mainly includes a hydraulic source 2, a reservoir 4, a servo valve 6, a relief valve 8, a fail safe valve hydraulic source 10, a fail safe valve reservoir 12, a solenoid valve 14 and a fail safe valve 16. The hydraulic source 2 supplies hydraulic fluid to a dual tandem hydraulic cylinder 18. A wall 22 is provided for a main body of the hydraulic cylinder 18. The wall 22 separates a space for hydraulic fluid supplied from the A system from a space for hydraulic fluid supplied from the B system. Flows of the hydraulic fluid from the A system and the B system are supplied to each other, thereby moving a piston rod 20 in the hydraulic cylinder 18.
A fail safe valve 16 has a structure with a spool valve 27 taking any of three states and small and large pistons for switching the three states. A first one of the three stats is a state that the hydraulic fluid is supplied from the hydraulic source 2 to the hydraulic cylinder 18 or returned from the hydraulic cylinder 18. A second one thereof is a state that the hydraulic source 2 stops the supply of hydraulic fluid to the hydraulic cylinder 18 when either the A system or the B system cannot operate due to a failure, and closes the hydraulic circuits between the fail safe valve 16 and the hydraulic cylinder 18 so that the hydraulic cylinder 18 may be moved by only a normally operating system. A third one thereof is a state that the hydraulic source 2 stops the supply of the hydraulic fluid to the hydraulic cylinder 18 and closes the hydraulic circuits between the fail safe valve 16 and the hydraulic cylinder 18 when both of the A system and the B system cannot operate due to a failure, and in addition, a flow of the hydraulic fluid is reduced by orifice. In the third state, the piston rod 20 performs a damping operation, since the flow of the hydraulic fluid is reduced even when external force is applied to the piston rod 20. Switching of the fail safe valve 16 is performed among the three states by supplying the hydraulic fluid to the small and large pistons of the fail safe valve 16 such that the spool valve 27 is switched by the fail safe valve hydraulic source 10.
Japanese Patent Application Publication (JP-P2001-295802A) discloses an electro hydrostatic actuator including a first position control system and a second position control system. The first position control system is a closed control system formed from a first operation section of the actuator, a position sensor for detecting the position of the first operation section, a controller, and an electric motor controlled by the controller to drive the hydraulic pump. The second position control system is a system which drives a second operation section for changing a displacement of the hydraulic pump in a direction of low displacement when a detection position signal outputted from the position sensor is coincident with a support position signal received by the controller.